• Choose a category
• All
• Classic-Fiction

By Root 622 0

new and often surprising consequences, allowing the theory to go well beyond general relativity. But these new phenomena are derived from the underlying principles and their realization; they are not by themselves added to the existing established framework. For string theory to be formulated consistently, it turns out that it requires more than just the three dimensions—height, depth, width—that we know from experience. It leads to many more kinds of particles than seen to date, and crucially rests on a new type of (super)symmetry relating different types of particles. So far, such concepts have been introduced for purely theoretical reasons. All this could be revolutionary, but it could also be wrong. The danger is that too much additional input, as is often required in specific models of string theory, can easily make the theory unpredictive if nearly everything can be encompassed in its breadth.

As for the problem of singularities and the big bang, both approaches recognize it as a crucial one and have made suggestions for potential solutions. But here, too, the vast scope of string theory does not allow for the situation to be as specific as one would like. There are very different proposals to address the singularity problem within string theory, such as the concept of our world colliding with another one embedded in a space-time of more than four dimensions (the ekpyrotic scenario developed by Justin Khoury, Burt Ovrut, Paul Steinhardt, and Neil Turok), the big bang as a collapsed but nonsingular and close-knit community of particles moving faster than light (the tachyon condensate introduced by Ashoke Sen, then used for cosmology by Eva Silverstein and Liam McAllister), or several versions of rebounding models somewhat similar to what we are going to see in loop quantum cosmology. In none of these cases, however, has a complete transition through the big bang singularity been achieved; assumptions about the transition are still necessary. This situation of a multitude of nonspecific scenarios makes it hard to see, at the current stage, how string theory might be able to deal with the singularity problem. Loop quantum gravity, on the other hand, directly addresses the structure of space-time, and does so with much greater (though still incomplete) clarity.

Before describing cosmological consequences, we must introduce more of the concepts of quantum gravity.

LOOP QUANTUM GRAVITY:

SHEDDING LIGHT ON DARKNESS

The history and development of loop quantum gravity is a fascinating one, a case study illustrating the process and perils of theoretical science. Several important contributions turned out to be incorrect, but nonetheless influenced many crucial developments. Enthusiasm was often followed quickly by sobering realizations of the limitations of what had been achieved. Faced with a vast unexplored area and fully aware of the importance of the subject, the initiators of the field demonstrated considerable courage. When applying traditional methods such as had been used for matter to a much more elementary concept such as space and time, one initially can hardly see any ground to stand on, nor any sign to show the way. Where do we start in such a situation, and where do we go from there? Every decision in these early stages is bound to have a lasting impression on the emerging field.

EARLY HISTORY: BRAIN GAMES

The complicated first five years of the theory were particularly interesting. Intellectually, the conditions for the few participating researchers were extreme. So many roads could be imagined, some ending in glorious success, most others in complete failure. Each of them would take years of calculations to follow, showing where it was leading only in the end; the first steps, the initial directions, thus had to be chosen with utmost care. Even for a theoretical physicist it is not easy to endure the corresponding double uncertainty of unknown scientific avenues combined with career insecurity.

In fact, not all theoretical physicists are created equal, and only a subclass is typically found at the